1. Field of the Invention
The present invention relates to an automatic focus control apparatus for detecting defocus amounts of a plurality of object areas within a frame of a photographic lens and for performing focus control of the photographic lens.
2. Related Background Art
In a conventional focus detection apparatus for a camera, the following method of detecting a defocus amount of an object is known well. Light beams emitted from an object and passing through different exit pupil areas of a photographic lens are focused on a pair of line sensors, and a relative positional displacement between a pair of image signals obtained by photoelectrically converting an object image is obtained, thereby detecting the defocus amount of the object.
In the above method, since a pair of focus detection systems (optical systems and sensors) are used, only a defocus amount in one object area within the frame is detected. However, various methods are proposed wherein a plurality of detection systems are prepared to detect defocus amounts of plural object areas within the frame.
In the latter methods, since plural object areas are used, a plurality of defocus amounts are detected. However, when the number of object areas to be focused in a camera is one or two (in the later case, focusing is performed by an intermediate value of the defocus amounts of the two object areas), an object area is selected in accordance with a given technique, and focusing of the photographic lens is performed by the defocus amount of the selected area.
As a method of selecting the object area, an object area judged as an object area nearest to the camera is generally selected.
The above selection method may have the following drawback.
In a single-lens reflex camera, an automatic focus control (AF) method includes a mode (to be referred to as a ONESHOT mode hereinafter) for locking focusing unless a release button is released once an in-focus state is obtained, and a mode (to be referred to as a SERVO mode hereinafter) for always performing focus control regardless of the in-focus or out-of-focus state. The ONESHOT mode is set when a photographer takes pictures of stationary objects, e.g., a portrait or scene. The SERVO mode is set when the photographer takes pictures of moving objects, e.g., a sport game or car race.
In portrait photographing dealing with a person as a main object, when an object area judged to be nearest to the camera is selected as in the above selection method, it is possible to focus on the person as the main object in most cases. However, as for a rapidly moving object as in sport photographing, the main object is usually present at the center of the frame in consideration of framing. When the nearest object area is simply selected, an object which is not the main object, e.g., spectators in a car race, may be undesirably set in an in-focus state.
Another method of selecting one of plural object areas is to select areas except for areas having improper sensor outputs. A typical example is a method of not selecting areas having low contrast levels.
The latter method is effective when sensor outputs are improper due to the low-contrast state. However, improper sensor output values may also be caused by dust attached to the sensors. In this case, a sensor to which dust is attached cannot be non-selected.
In addition, in the method wherein an improper sensor output is not selected to be used, when sensor outputs become improper as outputs for performing focus detection every time focus detection is performed, an unstable AF operation may occur.